The invention relates to a low-chlorine multistage process for continuous chlorine-free preparation of cycloaliphatic diisocyanates, which comprises the synthesis of diaminodiphenylalkanes, the hydrogenation thereof to the corresponding cycloaliphatic diamines and the subsequent conversion of cycloaliphatic diamines to the corresponding cycloalkylenebiscarbamates, and the thermal cleavage of the latter to the cycloaliphatic diisocyanates and alcohol.
It is known that diaminodiphenylalkanes (which also include substituted diphenyls and systems with fused aromatic rings (bicyclic or tricyclic ring systems)) can be prepared by condensation of an aromatic amine, for example aniline and aldehyde, over acidic catalysts. In the context of this invention, diaminodiphenylalkanes are also referred to as aromatic diamines.
The reaction is effected in such a way that N-alkyl compounds are first formed from an aromatic amine (aniline) and aldehyde. These precondensates then react further in the presence of acidic catalysts to give animals. These animals are subsequently rearranged under the action of an acidic catalyst to give diaminodiphenylalkanes.
In the prior art, the preparation of diaminodiphenylalkanes from the condensation of aromatic amines and aldehyde, especially aniline and formaldehyde, is frequently carried out. According to the reaction variant, either the condensation product of aniline and formaldehyde is first prepared and then rearranged in the presence of acids, for example hydrochloric acid, or else the condensation has already been carried out in the presence of acids under rearrangement conditions.
One disadvantage of this method is that salt-containing wastewaters are obtained in the homogeneous catalysis with mineral acids, especially with hydrochloric acid, and arise in neutralization of the acids. The chlorine salts are considered to be particularly critical. Furthermore, the aqueous mineral acids lead to corrosion problems in the production plants. Processes have therefore been developed in the further prior art in which appropriate heterogeneous catalysts are used. In addition to acidic ion exchangers, it is also possible to use acidic synthetic or natural silicas or aluminas, such as zeolites or clay minerals.
In U.S. Pat. No. 4,294,981, in such a process, the condensation is performed in the presence of a strong aqueous acid, after which the acid is removed by solvent extraction. The rearrangement is again carried out in the presence of strong acid which is used in a smaller amount. Diatomaceous earth, clays or zeolites can be used as the catalyst in this reaction stage.
DE-A-12 30 033 describes a process for preparing diaminodiphenylalkanes. This process uses silicon-containing clay, a synthetic silicon dioxide-aluminium oxide catalyst or a magnesium oxide-aluminium oxide catalyst.
A further reaction process for preparing diaminodiphenylalkanes is described in DE-A-14 93 431. This uses silicon dioxide, silicon dioxide-aluminium oxide or acid-treated aluminium oxide as a catalyst. Preference is given to silica gel or bentonite-type clay which contains silicon dioxide and aluminium oxide and is preferably acid-activated.
U.S. Pat. No. 4,071,558 describes a preparation process for preparing diaminodiphenylalkanes, in which an acid-activated clay catalyst, a silicon dioxide-aluminium oxide-containing cracking catalyst or a silicon dioxide-magnesium oxide catalyst is used.
U.S. Pat. No. 4,039,580 describes a preparation process in which the condensation of aniline and formaldehyde is performed in the absence of a catalyst and the condensation product is then reacted further in the presence of diatomaceous earth, clays or zeolites to give the diaminodiphenylmethane. Similar reactions are also described in U.S. Pat. No. 4,039,581.
The catalysts from the group of magnesium oxides or aluminium oxides, clay catalysts or silicon dioxide catalysts have not become established owing to their high costs, the low activities, the inhomogeneous quality and inadequate catalyst lifetimes.
The prior art therefore proposes, as a catalyst for preparing diaminodiphenylalkanes, an ion exchanger which possesses acidic groups. For instance, EP 0 043 933 A1 describes a process for preparing polyamine mixtures with a high proportion of 4,4′-diaminodiphenylmethane and a low proportion of 2,4′-diaminodiphenylmethane, in which the catalyst used is an ion exchanger based on a divinylbenzene/styrene copolymer. This ion exchanger possesses sulphonic acid groups, a specific surface area of 2 to 40 m2/g and a pore width of 0.5 to 40 nm. This wide range is demonstrated in the examples only by examples with pore widths of 1 nm. The acidic groups used for the catalyst are sulphonic acid groups. The yields in this process are in the range of 60 to 78%. Using the sulphonated styrene-divinylbenzene copolymer catalyst, it is possible to prepare diaminodiphenylmethanes which possess a high content of 4,4′-diaminodiphenylmethane. This isomer is especially required for further processing, specifically for conversion to corresponding diisocyanates of the diphenylmethane series, which constitute the starting materials in the preparation of polyurethanes or are used as coating raw materials. The publication further states that the proportion of 2,2′- and 2,4′-diisocyanatodiphenylmethane compounds has to be at a minimum because these isomers are undesired for many fields of application in the polyisocyanate sector. According to the prior art of EP 0 043 933, the resulting diaminodiphenylmethane compounds are subjected immediately to a phosgenation in order to prepare corresponding diisocyanates.
The process for preparing diaminodiphenylalkanes described in EP 0 043 933 has the disadvantage that it possesses low yields and, in spite of a high reaction temperature, very long reaction times are needed in order to achieve an industrially acceptable yield. A further disadvantage is that only a small proportion of 2,4′-isomer forms in the prior art process.
In addition to aromatic isocyanates, the corresponding aliphatic isocyanates are of particular significance in some specific fields.
The next stage in the preparation process of aliphatic isocyanates is the hydrogenation of the aromatic ring of the diaminodiphenylalkanes.
The hydrogenation of diaminodiphenylmethane (MDA) forms, from the 4,4′-isomer, 4,4′-trans/trans-, -cis/cis- and -cis/trans-diaminodicyclohexylmethane (PACM). The content of trans/trans-4,4′-diaminodicyclohexylmethane has a considerable influence on the crystallization tendency of the diisocyanate. When the trans/trans-4,4′ fraction of the product is too high in the diisocyanate prepared from PACM by phosgenation or other processes, the diisocyanatodicyclohexylmethane may form crystals even at room temperature, which is a hindrance for the further processing to polyurethanes. Before the further processing, complicated process steps therefore have to be undertaken in order to reduce the content of 4,4′-isomer to an acceptable value, such that crystal formation no longer occurs. This is typically done by enriching the 2,4′-isomer.
A further requirement on the isomer content in the preparation of diaminodiphenylmethane is that a very low proportion of 2,2′-isomer must be present, since this isomer causes chain termination in the polymerization reaction in the later process step to polyurethanes.
In order to avoid this additional complexity, it is important for this reaction route that a particular isomer ratio is achieved as early as in the preparation of diaminodiphenylmethane.
According to the prior art to date, this isomer ratio is obtained by purifying and distilling the diaminodiphenylmethane in a complicated manner in order to be able to provide the isomers in the ratio needed for the further processing.
It is known that cycloaliphatic amines having one or more amino groups can be prepared by catalytic hydrogenation of the corresponding mono- or polycyclic aromatic amines having one or more amino groups and optionally further substituents.
For the hydrogenation, catalysts based on ruthenium (EP 1 366 812, DE 15 93 293, DE 19 48 566, EP 0 001 425), rhodium (EP 0 630 882, EP 66 212) or mixtures thereof (U.S. Pat. No. 5,545,756, EP 0 231 788) are frequently used.
In the hydrogenation of diaminodiphenylmethane, referred to hereinafter simply as MDA, to methylenedicyclohexyldiamine, referred to hereinafter simply as H12MDA, especially formamide (EP 1 604 972) and MDA polymers (EP 1 149 629, EP 0 335 336) lead to a deactivation of the catalysts. When chloride is present in the MDA, it is absorbed by the catalyst and leads subsequently to increased formation of undesired polycyclic compounds (EP 0 324 190). The chloride has to be removed at regular intervals by washing the catalyst with water. This washing operation with water leads firstly to production shutdowns. Secondly, the reaction system subsequently has to be very substantially freed of water in order to minimize the formation of undesired hydroxyl compounds.
The synthetic access to isocyanates may be via a number of different routes. The oldest variant for industrial scale preparation of isocyanates, which is still prevalent to date, is the so-called phosgene route. The basis of this process is the reaction of amines with phosgene. The disadvantage of the phosgene process is the use of phosgene which, owing to its toxicity, its corrosivity and the high chlorine content, places particularly high demands on its handling on the industrial scale.
Process technology approaches which allow the use of phosgene for preparation of isocyanates to be avoided in industrial orders of magnitude are known. The term “phosgene-free process” is utilized in connection with the conversion of amines to isocyanates using alternative carbonylating agents, for example urea or dialkyl carbonate (EP 0 018 586, EP 0 355 443, U.S. Pat. No. 4,268,683, EP 0 990 644).
The basis of the so-called urea route is the urea-mediated conversion of diamines to diisocyanates via a two-stage process. In the first step, a diamine is reacted with alcohol in the presence of urea or urea equivalents (e.g. alkyl carbonates, alkyl carbamates) to give a diurethane, which typically passes through an intermediate purification stage and is then cleaved thermally in the second step to diisocyanate and alcohol (EP 0 355 443, U.S. Pat. Nos. 4,713,476, 5,386,053). Alternatively, the actual urethane formation may also be preceded by the separate preparation of a diurea by controlled reaction of the diamine with urea (EP 0 568 782). Also conceivable is a two-stage sequence consisting of partial reaction of urea with alcohol in the first step and subsequent metered addition and urethanization of the diamine in the second step (EP 0 657 420).
The thermal cleavage of urethanes to the corresponding isocyanates and alcohols has been known for some time and can be performed either in the gas phase at high temperatures or at relatively low temperatures in the liquid phase. However, a problem in both procedures is that, as a result of the thermal stress, undesired side reactions always also occur, which firstly reduce the yield and secondly lead to the formation of resinifying by-products which considerably disrupt the course of an industrial process as a result of depositions and blockages in reactors and workup apparatus.
There has therefore been no lack of attempts to achieve yield improvements by chemical and process technology measures, and to limit undesired by-product formation. For instance, a series of documents describes the use of catalysts which accelerate the cleavage reaction of the urethanes (DE 10 22 222, U.S. Pat. No. 3,919,279, DE 26 35 490). In fact, it is entirely possible in the presence of suitable catalysts—which are a multitude of basic, acidic and organometallic compounds—to increase the isocyanate yield compared to the uncatalysed variant. However, the formation of undesired by-products cannot be avoided even by virtue of the presence of a catalyst. The same applies to the additional use of inert solvents, as likewise recommended in U.S. Pat. No. 3,919,279 and DE 26 35 490, in order to ensure homogeneous distribution of the heat supplied and of the catalyst in the reaction medium. In principle, the use of solvents which boil under reflux, however, has the consequence of a reduction in the space-time yield of isocyanates and is additionally afflicted with the disadvantage of an additional high energy demand.
The examples of thermally catalysed cleavage of monourethanes adduced in EP 0 054 817 describe the partial discharge of the reaction mixture to remove the resinifying by-products which form in the course of urethane cleavage. This procedure serves to prevent depositions and blockages in reactors and workup equipment. There are no indications to a yield-enhancing utilization of the partial discharge. EP 0 061 013 describes a similar approach to a solution, wherein the thermolysis in this case is performed in the presence of solvents whose task apparently consists in a better absorption of the nonvolatile by-products. Here too, the partial discharge is not utilized for the purpose of optimizing the yield.
EP 0 355 443 discloses that an increase in the yield can be achieved when the relatively high molecular weight, utilizable and unutilizable by-products formed during the cleavage of diurethanes in the cleavage reactor, to ensure a disruption-free and selective reaction, are discharged very substantially continuously from the reactor and then converted for the most part in the presence of alcohol, and then recycled into the diurethane preparation. The procedure described is associated with a high energy demand, since unutilizable by-products are removed by distillation from the effluent of the diurethane preparation, for which the entire diurethane has to be evaporated. In contrast to EP 0 355 443, the urethanization effluent, in the process of EP 0 566 925, is divided into two substreams of which only one is freed by distillation from its high-boiling, unutilizable by-products, before the combined diurethane streams are fed to the deblocking reaction in the cleavage reactor. In addition, the continuous cleavage reactor discharge in EP 0 566 925 is recycled directly, i.e. without a reurethanization step, into the diurethane synthesis.
The procedure of EP 0 566 925 has the consequence that a portion of the high boiler components from the diurethane synthesis passes via the deblocking stage back into the diurethane preparation and further into the diurethane purification procedure.
Possible processes which do not have the disadvantages of the prior art detailed, also guarantee good plant availability and a good process yield over the long term and additionally allow the saving of investment and energy costs are described in EP 1 512 680, EP 1 512 681, EP 1 512 682, EP 1 593 669, EP 1 602 643, EP 1 634 868.
The commercial form of industrially produced urea which is now prevalent is that of prills, i.e. small spheres having a diameter of 1-3 mm. Crystalline urea has such a great caking tendency even at very low water contents of <0.1% that it is not an option for loose storage in large amounts. An improvement in the storage properties of urea prills, which appears to be necessary, for example, in the case of silo storage of large amounts, is achieved by a subsequent surface treatment of the prills with powder substances, for example talc, bentonites, kieselguhr, diatomaceous earth or other silicatic substances, or by means of sulphur, and also by spray application of small amounts of oil. In this connection, reference is also made to conditioned urea.
Nowadays, the urea industry (Ullmann's Encyclopedia of Industrial Chemistry, Release 2006, 7th Edition) preferably adds up to 0.6% by weight of formaldehyde to the urea melt before the prilling, in order to increase the stability of the prills. This measure serves for prevention of decomposition and caking in the course of transport and for improvement of the storage stability.
The processes of EP 1 512 680, EP 1 512 681, EP 1 512 682, EP 1 593 669, EP 1 602 643, EP 1 634 868 in principle also allow the use of conditioned urea, or else any mixture of conditioned and unconditioned urea. However, preference is given to using unconditioned urea which has thus not undergone any subsequent surface treatment which serves for storage stability. The urea can be used in various administration forms (prills, granule, crystals, melt, solution).
It was a general object of the present invention to find a process for preparing cycloaliphatic diisocyanates, especially dicyclohexylmethane diisocyanate (H12MDI), which, proceeding from the aldehyde and aromatic amine starting materials, via the intermediates of an aromatic diamine and of a cycloaliphatic diamine, works in an ecologically and economically optimized manner. At the same time, the process should as far as possible be performable without chlorine and without chlorine-containing chemicals as feedstocks and substances formed in the process, for instance hydrochloric acid or chlorides such as sodium chloride. The use of phosgene should be dispensed with. Moreover, the conversion rates (yield) of the particular feedstocks to the particular end products, taking account of the particular advantageous isomer distribution, should be high.
It was another technical object of the invention, in a first stage, to provide a process for preparing diaminodiphenylalkanes, which leads directly to the specific isomer ratio which is required for the further processing to the cycloaliphatic diamine and the corresponding diisocyanates. It is a further technical object of the invention to develop a process which works more economically viably, because it requires a shorter reaction time and leads to the desired yields of the isomers. At the same time, the formation of wastewater with a salt burden, especially of chlorine-containing salts, should be at a minimum.
The object is achieved by the present low-chlorine multistage continuous process as detailed below.